The physics and philosophy of time - roughly a 1 hour lecture by Carlo Rovelli.
It's a conceptual overview. It got very interesting to me at about 20 - 25 minutes in and then stayed interesting 'til the end.
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and use it to measure the height of a mountain.
>"According to Einstein’s theory of relativity, time moves differently depending on where you are in a gravity field.
For example, a clock on top of a tall mountain will move a tiny bit faster than a clock at the base of that mountain, where the gravity is stronger.
It’s not a mechanical error. Time itself actually passes faster at the top of the mountain."<
https://www.washingtonpost.com/national/health-science/an-atomic-clock-is-used-to-measure-not-time-but-the-height-of-mountains/2018/02/23/5a845166-11c3-11e8-9570-29c9830535e5_story.html?noredirect=on&utm_term=.fb7e357758ab
I did not know this. 
Hmmm... I lived in Truckee for a few years. Elevation 6,000. Maybe I'm an older 77 than I thought. sigh.
burnt at both ends in my youth
